When planning the irradiation of a patient in the context of radiation therapy, radiological data is captured via e.g. CT image recordings so that the radiation dose for the planned irradiation can be established. In particular, it is important to establish radiation doses with a high degree of spatial resolution, in order to destroy only malign tissue in the region to be irradiated and to preserve adjacent and possibly very sensitive regions.
The interactions occurring between radiation and tissue during the irradiation can be divided into primary and secondary effects. The primary effects are the direct interaction of the radiation with the tissue. In the case of irradiation with photons, the interaction is primarily with electrons. If tissue is irradiated with heavy particles, the interaction is mainly with the atomic nuclei. In addition, in the case of the primary processes described above, so much energy is transferred to the electrons during the interaction that these are separated from the molecule and still have enough energy themselves to cause further ionization processes as a secondary effect. Different effects occur during the interaction of electromagnetic radiation with electrons. The Compton effect is dominant when radiation is absorbed in the soft tissue, this consisting mainly of water, whereas the photoelectric effect is dominant in the case of absorption in solid body matter, such as e.g. bone matter.
In order to be able to determine the radiation dose for radiation therapy in advance, it is necessary to know the charge density distribution, i.e. in particular the electron density distribution or the nuclear charge distribution of the material that is present in the region to be examined.
A conventional method for determining electron densities on the basis of CT image data records consists in mapping attenuation values of the CT image data, also referred to as CT values below, onto electron densities using a simple table. However, a very high level of accuracy is not achieved using this method, because when applying polychromatic X-radiation as used in the case of CT image recordings, CT values of the same material in the image are dependent on the size of the examined object in which they are absorbed, and also dependent on the position of the irradiated region in the cross section of the object.
This stems from the fact that, due to the radiation hardening, a near-surface volume element is exposed to a softer radiation than a centrally situated volume element during the mapping. For the same density and the same material, the near-surface volume element is therefore assigned a higher CT value (greater degree of attenuation) than the centrally situated volume element. Due to the different CT values, the near-surface volume element is therefore assigned a higher electron density than the centrally situated volume element. The accuracy of this method is therefore limited even if a calibration is performed very accurately and repetitively in advance using a test body (so-called phantom).
Another way of determining electron densities is based on the CT measurement using two spectra, also known as dual-energy CT, wherein the recorded measurement data is represented in a basic material breakdown. The measurement data divided according to individual materials can then be mapped onto electron densities again. As explained above, the absorption properties of the biologically relevant materials are essentially based on only two different effects, the photoelectric effect and the Compton effect, and therefore a breakdown of the measurement data according to two basic materials, e.g. water and calcium, is sufficient. In this way, the influence of the patient size and the position of a volume element in the body of the patient is reduced for these materials.
However, not all CT devices have the possibility of a dual-energy image recording, and therefore this method has limited availability.